How do I convince my mom to let me buy razors?

I am thirteen, and have an overprotective mom of sorts. She is always questioning every little thing I do. I understand it is for my safety,but sometimes it is a little ridiculous. Well, I would like to get some razors, and shave for personal hygeine. However the only time I could go to a store is if she drives me, and when I would walk up to the cashregister with some razors she would ask what I need them for, and things would go south from there. I am very .... 'developed' so the cashiers would think nothing of it, my mom, knowing my true age would just flip out though. I don't know if I left anything out, but just help me find a way to convince her.